Gas analyzing instrument



L. A. DlETZ GAS ANALYZING INSTRUMENT 2 Sheets-Sheet 1 Filed Oct. 29,1954i J27 van to)" [I eonaro fl 3/95} y Mu W 'f/i's fitor'ney UnitedStates Patent O GAS ANALYZING INSTRUMENT Leonard A. Dietz,Pattersonville, N. Y., assignor to General Electric Company, acorporation of New York Appiication October 29, 1954, Serial No. 465,531

4 Claims. (Cl. 250-413) The present invention relates to gas analyzinginstruments.

More particularly, the invention relates to mass spectrometer type gasanalyzing instruments used primarily for detecting tracer gasesintroduced into pressure or vacuum systems for leak detection purposes.

Gas analyzing instruments of the above-identified type have beenavailable to industry for a number of years, and for a description of aparticular construction of one of these instruments, reference is madeto U. S. Patent No. 2,504,530--R. B. Jacobs, issued April 18, 1950,Vacuum Leak Detector Method. While the presently known gas analyzinginstrument constructions have been suitable in many respects, they arenot entirely satisfactory for a number of reasons' The most importantfailure of existing constructions is their relatively high cost andcomplexity of design. A second undesirable characteristic of knowninstrument constructions is the ditficulty experienced in initiallyaligning the various component parts of the instrument, and inmaintaining such alignment under operating conditions.

It is, therefore, one object of the present invention to provide a massspectrometer gas analyzing instrument which is simple in design andrelatively inexpensive to manufacture.

Another object of the invention is to provide an instrument of the abovetype wherein the component parts can readily be aligned at the time theinstrument is initially placed in operation, and wherein such alignmentcan be maintained with a minimum of efiort.

A further object of the invention is to provide a mass spectrometer typegas analyzing instrument which incorporates the above features but whichhas comparable or better operating characteristics than existinginstruments of the same type.

In practicing the invention, a mass spectrometer gas analyzinginstrument is provided wherein a separate analyzer tube is not required,and all essential components of the instrument are mounted on a singlesupporting member that can readily be assembled in vacuum tightrelationship Within a housing capable of being evacuated to a highdegree. In a preferred embodiment of the invention a housing is providedwhich is designed to be maintained under a high vacuum. An ion source issupported within a housing and a means is made available for introducinga gas sample to be analyzed. into the ion source. A pair of magnets aresecured within the housing and are positioned on opposite sides of theion beam path produced by the ion source and cause the ions in the ionbeam to be separated out into different characteristic ion beams havingdistinct mass-to-charge ratios. A collector assembly is also securedwithin the housing and is disposed in the ion beam path for deriving anoutput 7 electric signal representative of the number of ions having adesired mass-to-charge ratio.

Other objects, features, and attendant advantages of .this inventionwill be appreciated more readily as the Patented Oct. 16, 1956 samebecomes better understood by reference to the following detaileddescription, when considered in connection with the accompanyingdrawings, wherein like parts are identified by the same referencecharacteristic and wherein:

Fig. 1 is a sectional view of a mass spectrometer type gas analyzinginstrument constructed in accordance with the present invention;

Fig. 2 is a partial cross-sectional view of the instrument shown in Fig.1, and illustrates the same in assembled relation with a suitable vacuumsystem therefor;

Fig. 3 is a broken-away perspective view of the instrument shown in Fig.1, and illustrates the details of construction thereof; and

Fig. 4 is a cross-sectional View of the ion source portion of theinstrument shown in Fig. 1.

The gas analyzing instrument shown in Fig. l of the drawings comprises abase member 11 which may be constructed of stainless steel or some otherconductive material, and, as is best seen in Fig. 2 or" the drawings, isadapted to form one wall of a housing 12 that is designed to beevacuated by a suitable vacuum system. Mounted on the base member 11 isan ion source 13 having an inlet tube 14, best seen in Figs. 3 and 4 ofthe drawings, for introducing a gaseous sample to be analyzed into theion source. Also mounted on base member 11 is a magnet 15 having a pairof spaced apart pole pieces 16 and 17 which are positioned on oppositesides of the ion beam path produced by the ion source 13. Also disposedin the ion beam path at a point farther away from the ion source 13 thanthe permanent magnets is a collector assembly 18.

The ion source 13 comprises a generally cylindrical block 21 having anopen sided hollow chamber 22 formed therein and having one exteriorsurface 23 thereof machined to present a flat rectilinear mounting, asis best shown in Fig. 3 of the drawings. The block 21 is supported onthe base member 11 by a plurality of insulating supports 24, and has theinlet tube 14 connected thereto for providing access into the hollowchamber 22 for the gases to be analyzed. An apertured plate 25 issecured to the open face of the block 21 that has a small slit aperture26 therein which defines an exit slit for the ions produced Within thechamber 22. Directly opposite the exit slit 26, a pusher electrode 27 issecured within the hollow chamber 22 by means or" an insulating supportthat has a central mounting post extending through the insulatingsupport to a point exterior of the chamber for providing electricalcontact thereto. Also secured to the block 21 are a plurality ofaccelerating electrodes formed by a pair of opposed semi-circular shapedplates 28 and 29 spaced apart a sufficient distance to accommodate theion beam, and a second plate 31 having a slit aperture centrallydisposed therein to provide a passageway for the ion beam. Each of theplates 28, 29 and 31 are sn ported on the block 21 by means of aplurality of insulating posts 32 threadably secured to the block 21 bystuds attached thereto.

Detachably secured to the flat side 23 of block 21 is an electronfilament assembly that comprises an electron emissive ribbon oftungsten, or other similar material supported between a pair of terminalposts 33. The posts 33 are mounted in a suitable insulating support 34,and have a reflector 35 attached to the end thereof in back of theelectron emissive ribbon. Electron emissive filament is aligned with anaperture 36 formed in the side of block 21, best seen in Fig. 4 of thedrawings, which allows passage of electrons emitted from the filament,and serves to shape the electrons into a thin pencil-like beam. Theelectron filament assembly is supported between, and aligned with, apair of permanent magnets 37 and 38 which are secured in opposite endsof a mounting post 41 secured in the base member 11.

In operation a gaseous sample to be analyzed is introduced into thehollow chamber 22 through gas inlet tube 14 where it is bombarded bytheelectron beam produced by the electron filament in cooperation withthe aperture 36 in the side of block 21. Bombardment of the molecules ofgas by the electrons results in the production of a number of positiveions in accordance with the wellknown electron bombardment phenomenon,and the ions thus produced are subjected to the combined influence ofthe pusher-electrode 27 with the accelerating electrodes 25, 28, 29, and31. The combined action of all of these electrodes causes the ionsformed in the chamber 22 to be shaped into an ion beam that passes outthrough the exit slits in the accelerating electrodes and on into theinfluence of the permanent magnet-17. The electrons which are notutilized in forming ions by direct impact with the molecules of gas,continue on across the chamber and discharge upon the walls of the block21 which is in effect at ground potential. To ensure proper sensitivityof the instrument and a speedy response to the changes in thecomposition of the gaseous sample introduced through the inlet tube 14,the ratio of the slit diameters of the exit slit in acceleratingelectrode 25 is adjusted so. thata differential pressure exists betweenthe chamber system introduced into the chamber 22 of source 14, acollector assembly 18 is positioned to intercept the ion beam at a pointsubsequent to the separation of the beam. This collector assemblycomprises a rectangularly-shaped boxlike shield 44 having one endthereof secured to base member 11 in cantilever fashion, and having anaperture 45 formed in the free end thereof for accommodating ions havinga desired mass-to-charge ratio. A collector electrode 46 is supportedwithin shield 44 on an insulator 47 and is positioned so asto interceptthe ion beam after passage throughaperture 45; and positionedintermediate the aperture 45 and the collector electrode 46 is anadditional electrode 48 to which a separate potential may be appliedfree end of the box-like structure 44to accommodate the.

desired ion beams. Also, if desired, an output amplifier such as thatshown at 49 in Fig. 2 may be mounted within and the acceleratingelectrode region having a ratio of modating the electron beam, highsensitivity of the ion source can be maintained. Also by positioning theelectron emitter outside of the high pressure region of theion source inthe manner illustrated, the region in the neighborhood of the filamentis evacuated to the same degree as the remainder of the housing andresults in prolonging the life of the filament. In addition, bypositioning the electron filament assembly in this manner, it can bemore readily removed thus simplifying maintenance of the instrument.

The first set of magnets formedcby the pole pieces 16 and 17 inconjunction with the permanent magnet 15, are supported on an angleplate 42 secured to the end of the mounting post 41. While theconstruction discloses a single magnet having two separate pole piecestocform the two sets of magnets, it is believed obvious that two sepa+rate and distinct permanent magnets, such as Alnico magnets, could besubstituted in place of the-arrangement shown. The spacedv apart polepieces 16 and 17 serve to develop an analyzing magnetic fieldtherebetween which acts upon the ion beam produced by the ion source13'to separate the ions therein into distinct ion beams havingcharacteristic mass-to-charge ratios. The action of the magnetic fieldis to cause the ions to make circular paths with radii having valuesgiven by the equation where R is the radius of the path in centimeters,

B is the magnetic field strength in Gauss, V

m is the mass of the ions expressed in atomic mass units on the physicalscale,

V is the energy of the ions in volts electrostatic units, and

k is the charge of the ions in the electrostatic units.

tracer gas used to detect leaks in-the vacuum or'pres'sure the box-likeshield 44, and a suitable output connection 50 provided for derivingoutput electric signals from the collector electrode 46.

The entire structure comprised by base member 11,

and the ion source 13, the analyzing magnets 16 and 17 andcollectoras'sembly 18 which are secured thereto, is

adapted to be supported within vacuum tighthousing 12 in the mannershown in Fig. 2 of the drawings.v For this purpose, the base member 11has a groove formed in one surface for accommodating .an O-ring gasketthat allows the base member to be supported onthe housing 12 in vacuumtight relationship. The housing 12 may comprise a stainless steel pipehavinga diameter in the neighborhood of and is connected through 'acheck valve 51, manifold 52, and suitable piping to a roughpump 53 forevacuating the entire housing structure to a high degree.

From a consideration of the foregoing description,

. it can be appreciated that 'all the essential components of a massspectrometer type gas analyzer instrument are supported within onesingle housing lthus simplifying greatly its construction andfacilitating alignment of the various component parts thereof, as wellas the maintenance of such alignments while the instrument is inoperation. Hence,'it can be appreciated that the invention provides anew and improved gas analyzing instrument simple in construction andrelatively. inexpensive to manufacture wherein the essential componentparts are supported within a single housing member adapted to beevacuated. By constructing theinstrument in this manner, the needfor anelaborate vacuum system, the need for. separate analyzer tube heretoforerequired in known instruments ofthis type, are done away with. Further,this type oficonstruction facilitates initial alignment of allof theessential parts of the instrument as well as maintenance of theinstrument while in operation. The above advantages are incorporated.inthe instrument without in any way sacrificing its sensitivity or speedof response when contrasted to the existing known instruments, and byproperly proportioning certain paraml. A mass analyzing instrumentcomprising a basemember, anion source supported on said base member,

means for introducing a gas to be analyzed into said ion source, a pairof magnets supported on said base member and positioned on oppositesides of the ion beam produced by said ion source for causing the ionsto be separated out into different ion beams having distinctmass-to-charge ratios, and a collector assembly secured to said basemember and disposed in the ion beam path, said base member and thecomponents mounted thereon being adapted for insertion in a singlehousing structure.

2. A mass analyzing instrument comprising a base member, an ion sourcesupported on said base member having an electron filament assembly,means for intro ducing a gas to be analyzed into said ion source, afirst pair of opposed magnets supported on said base member so as to beon opposite sides of said ion source and aligned with said electronfilament assembly for collimating the beam of electrons produced by saidfilament assembly, a second pair of magnets supported on said basemember and positioned on opposite sides of the ion beam produced by saidion source for causing the ions to be separated out into different ionbeams having distinct mass-to-charge ratios, and a collector assemblysecured to said base member and disposed in the ion beam path, said basemember and the components mounted thereon being adapted for insertion ina single integral housing.

3. A mass analyzing instrument comprising a base member, an ion sourcesupported on said base member having an electron filament assembly,means for introducing a gas to be analyzed into said ion source, a firstpair of opposed magnets supported on said base member so as to be onopposite sides of said ion source and aligned with said electronfilament assembly for collimating the beam of electrons produced by saidfilament assembly, a second pair of magnets supported on said basemember and positioned on opposite sides of the ion beam produced by saidion source for causing the ions to be separated out into different ionbeams having distinct mass-to-charge ratios, a box-like shield structuresecured to said base member in cantilever fashion, and a collectorelectrode insulatingly supported on the free end of said shieldstructure and disposed in the ion beam path, the free end of said shieldstructure having an aperture therein for accommodating the ion beam andsaid base member with the elements mounted thereon being adapted forinsertion in a single housing structure.

4. A mass analyzing instrument comprising a base member, an ion sourcesupported on said base member and having an electron filament assemblysecured to one side of said ion source, said ion source having anaperture therein for accommodating a beam of electrons produced by saidfilament assembly, a first set of magnets aligned with the electronfilament assembly for collimating the electron beam, said first set ofmagnets being supported on the opposite ends of a yoke, a post havingone end secured to said base member in cantilever fashion and havingsaid yoke secured thereto intermediate the ends thereof, a second set ofmagnets secured to the free end of said post and positioned on oppositesides of the ion beam produced by said ion source, a box-like shieldstructure secured to said base member in cantilever fashion, and acollector electrode insulatingly supported Within the free end of saidshield structure and disposed in the ion beam path, the free end of saidshield structure having an aperture therein for accommodating the ionbeam.

References Cited in the file of this patent UNITED STATES PATENTS2,341,551 Hoover Feb. 15, 1944 2,355,658 Lawlor Aug. 15, 1944 2,511,728Long June 13, 1950 2,537,025 Berry Jan. 9, 1951 2,551,544 Nier et al.May 1, 1951 2,587,647 Pallette Mar. 4, 1952 2,624,845 Thompson Jan. 6,1953

